1. Field of the Invention
The field of this invention is displacement washing of porous media. More particularly, the invention involves washing and bleaching of pulp fiber mats.
2. Description of the Prior Art
In the manufacture of many materials, particularly chemicals, it often becomes necessary to separate the components of a mixture by dissolving one or more of them from the processing mixture. A common process requires washing the mixture with a solvent in which one or more of the materials constituting the mixture is soluble and the material sought to be recovered is insoluble.
In pulp and paper manufacture, it is often necessary to wash the pulp to remove contaminants or recover processing chemicals. For example, after wood chips are digested in a chemical solution designed to dissolve lignin, thereby freeing the wood pulp fibers, it is necessary to separate the lignin bearing spent cooking liquor from the pulp. Recovery of the liquor is important economically as a means of recovering cooking chemicals and environmentally as a means of preventing water pollution.
Black liquor recovery or brownstock washing, as it is usually called, is most often accomplished by rotary-drum vacuum filters. These filters are generally arranged in a series of four, with countercurrent flow of wash fluid. A typical rotary-drum vacuum washer comprises a wire-cloth cylinder partially submerged in a slurry tank of fibers. The cylinder is provided with a vacuum which, as the drum rotates, causes the pulp to build up on the wire. A number of showers are located near the drum surface to direct wash solution onto the pulp as it travels about the drum. The pulp at a selected consistency then discharges from the drum to the next washing stage for subsequent processing.
The cost of energy and stringent effluent controls emphasize the need to minimize dilution of recovered chemicals during brownstock washing while maintaining a high level of chemical recovery. In many cases poor washing efficiency may be limiting production and thus the incentive to improve efficiency is high.
In commercial brownstock displacement washers, the displacing wash solution is usually a more dilute solution of the spent cooking liquor that is sought to be displaced from the fiber pad. There is appreciable mixing within the pulp and fiber matrix between the liquor and wash fluid during washing, primarily as a result of preferential penetration of the wash fluid in regions of the pad having higher permeability. Fingers or channels of penetrating wash fluid form in these areas of pad nonuniformities. These "fingers" have lower pressure drops along their length due to the lower viscosity of the wash fluid with respect to the liquor to be displaced. The lower pressure drop promotes growth of the fingers. As a result of the development of the fingers or channels of wash fluid, large regions of stagnant liquor are eventually by-passed. This phenomemon, termed herein "viscous fingering," is the primary mechanism adversely influencing the effectiveness of brownstock displacement washing.
The viscous fingering phenomenon was first described in relation to secondary recovery of petroleum by water or brine injection into oil bearing strata. As has long been practiced in the oil industry, recovery is improved by displacing the oil from its porous formation with water. If the natural pressure of the formation were relied on alone, a significant amount of oil would otherwise remain locked in the ground. Researchers found that one of the problems encountered in water injection was a tendency for the water to preferentially pass through larger pore openings of the oil bearing rock, bypassing the smaller passageways. As a consequence, the water or brine would tend to "finger" through the strata and bypass oil. In a search for injectable liquids that could do a better job of displacing oil, prior workers tried to match the "mobility" of the displacing fluid to that of the oil. Mobility is defined as a characteristic of a fluid in certain media related to the piston like capability of the fluid to displace a solution retained in media without the displacing fluid channeling through the solution-media.
A low viscosity fluid such as water or brine injected into a porous strata typically was discovered to have a higher "mobility" than the more viscous oil. Early attempts to use such materials as glycerine, sugar or glycols failed on the basis of economics. Workers in the field of secondary oil recovery subsequently discovered that certain polymers dissolved in water at low concentrations impart a reduced mobility to injected water with respect to the oil sought to be displaced. These polymers exhibit a far greater impact on the mobility of water in rock formations than the measured solution viscosity in a laboratory would indicate. See Jennings, U.S. Pat. No. 3,687,199.
The unusual ability to improve oil recovery is a property of only a few select water-soluble polymers. Among these are the extensive family of acrylamide polymers and copolymers. The use of certain of these high weight polymers in injection waters, particularly synthetic, partially hydrolyzed polyacrylamide solutions, substantially improves secondary and tertiary oil recovery, as is described in the petroleum recovery literature. See Pye, "Improved Secondary Recovery by Control of Water Mobility," J. Pet. Tech. 911, (August 1964).
Very dilute solutions of these particular polymers, at 250-500 ppm exhibit viscosities differing only slightly from water. However, flow of these solutions through a microporous medium such as sandstone may exhibit 5-10 times more displacement capacity than other solutions having much higher viscosities. Such performance is exhibited only in porous media having a pore structure that forces displacing fluids to travel a tortuous path in passing through them.
In Pye, Id. at 911, Darcy's law, describing the flow of fluids through porous media, is expressed as ##EQU1## wherein q=flow rate (ft.sup.3 /seconds); .DELTA.p=pressure drop (lbs/ft-sec..sup.2); A=area (ft.sup.2); L=thickness of the medium (ft); .mu.=fluid viscosity (lb/sec-ft.sup.2); and k=permeability of the media to the fluid (ft.).
Fluid viscosity and its interaction with the permeability of the medium are characteristics which function as resistance factors to flow. The ratio of permeability to viscosity (k/.mu.) for a particular system is often called the mobility, .lambda., of a fluid with respect to a particular medium.
The ratio of the mobility of a solvent to the mobility of a solution of the solvent containing a polymer, under equivalent conditions of fluid saturation and temperature has been termed by Pye the "resistance factor," R of the polymer solution. The resistance effect of a polymer where water is the solvent, is expressed in terms of mobilities, as: ##STR1##
As polymer is added, the mobility .lambda..sub.p =k.sub.p /.mu..sub.p of the displacement solution, flowing through the particular porous medium, is reduced. Consequently, the resistance factor of a system, that is, the flowing properties of the solution without the polymer relative to the flowing properties of the solution with the polymer, is increased. The observed effect of the polymer addition is that the treated solution does a significantly improved job of displacing a viscous fluid retained in a porous medium than an untreated displacing solution. The ability of a displacing wash liquid to displace a viscous liquor from a porous medium increases as its mobility with respect to that porous medium is reduced.